The ability to eliminate waste heat is an important feature of any power generation or power transmitting system, and particularly of space-based power generation or power transmitting systems. Eliminating waste heat is also an important need for human or robotic activity in space. Both nuclear and concentrator types of solar space-based sources of electricity result in the production of several times as much unusable power as is converted to electricity. The only continuous method of eliminating this excess energy in space is by radiation. The high cost of lifting payloads to orbit necessitates thermal management systems with the minimum mass possible.
The rate of radiation heat transfer is proportional to (T4hot−T4cold), where generally Thot>>Tcold. Hence, having the maximum Thot possible for a given system is desirable. However, Thot is determined by the specific power generation process that is used. For example, solar cells are presently limited to operate at a maximum cell temperature below 100° C. If concentrator cells are used, the excess energy is much greater than the amount that can be radiated directly by the cells at these temperatures so an auxiliary radiator is typically needed. Nuclear power systems operate at much higher temperatures, so the materials used in the radiator need to withstand these higher temperatures.
The internal transfer of energy from the waste heat source to the radiators can only be accomplished three ways: (1) conduction across a temperature gradient (through solid, liquid, or gas); (2) single-phase pumped fluid (gas or liquid) moving between different temperatures; or (3) two-phase fluid in a heat pipe using the heat of vaporization and condensation. All three of these methods may be used in space, with each method having advantages and limitations. If the distance from the heat source to the radiator is very small, method (1) may be a desirable method. As working distances increase, (2) and (3) may become more desirable choices. Modest sized cooling systems often use heat pipes since they are reasonably low mass for a given power level, and do not require a large temperature change to transport the energy.
In space heat-pipe systems, capillary action (i.e., wicking), feed gas pressure, or an external pump are typically used to return the condensed liquid (unless the system is in a rotating system using centrifugal force to return the condensed liquid). If the systems are small enough, wick return may be an acceptable method. However, wick return or direct pumping of the condensed liquid by the gas within the condenser limits the liquid return rate and the tied up liquid caused by the limited return rate adds to the total mass (and therefore the cost to lift into space) of these systems. The use of external pump return (open loop) has been previously limited due to the difficulty of isolating and collecting the condensed liquid.